1. Field of the Invention
This invention relates to steam generators and water heaters, also referred to herein as steam boilers and hot water boilers. More particularly, this invention relates to space-efficient steam generators and water heaters having improved energy efficiency over conventional steam generators and water heaters. The improved energy efficiency is achieved by recovering both the sensible and latent heat of vaporization from moisture in the flue gases and returning the recovered energy to the steam generator or water heater. In addition to the energy efficient steam generators and water heaters, this invention relates to space-efficient steam generators and water heaters having reduced NOx emissions over conventional steam generators and water heaters.
2. Description of Related Art
Many industrial processes produce process streams containing condensable components such as water vapor. As the mere discarding of these condensable components can constitute a substantial loss in available heat energy, it is desirable to recover these condensable components from the process streams for economic reasons. It is also desirable to recover the latent heat of vaporization associated with such condensable components as a means for reducing process energy requirements. The use of heat exchanger-based condensers for the recovery of condensable components of process streams and the latent heat of vaporization associated therewith is well known to those skilled in the art.
Methods and apparatuses for the selective removal of one or more components from a gaseous mixture are well known. U.S. Pat. No. 4,875,908 teaches a process for selectively separating water vapor from a multi-component gaseous mixture in which the multi-component gaseous mixture comprising the water vapor is passed along and in contact with a membrane which is selectively permeable to water vapor. The use of membranes for selective removal of one or more components of a gaseous mixture is also taught by U.S. Pat. No. 4,583,996 (inorganic porous membrane), U.S. Pat. No. 3,980,605 (fibrous semi-permeable membrane) and U.S. Pat. No. 3,735,559 (sulfonated polyxylylene oxide membranes).
Methods and apparatuses for selective removal of water vapor from a gaseous mixture and condensing the separated water vapor to recover its latent heat of vaporization are also known. U.S. Pat. No. 5,236,474 and related European Patent Application 0 532 368 teach a process for removing and recovering a condensable vapor from a gas stream by a membrane contactor in which a gas stream containing a condensable vapor is circulated on one side of hollow fiber membranes while cool extraction fluid is circulated on the other side under a total pressure differential. As a result, the condensable vapor in the gas stream is condensed in the gas stream and the condensed vapor, i.e. liquid, permeates the membrane and becomes entrained in the cool extraction fluid.
U.S. Pat. No. 4,466,202 teaches a process for recovery and reuse of heat contained in the wet exhaust gases emanating from a solids dryer or liquor concentrator by preferentially passing the vapor through a semi-permeable membrane, compressing the water or solvent vapor, and subsequently condensing the water or soluble vapor in a heat exchanger, thereby permitting recovery of its latent heat of vaporization for reuse in the evaporation process. It will be apparent to those skilled in the art that a substantial amount of energy will be required to compress the water or solvent vapor in accordance with the process of this patent. U.S. Pat. No. 5,071,451 teaches a vapor recovery system and process that permits condenser vent gas to be recirculated. The system includes a small auxiliary membrane module or set of modules installed across a pump and condenser on the downstream side of a main membrane unit, which module takes as its feed the vent gas from the condenser and returns a vapor-enriched stream upstream of the pump and condenser.
FIGS. 1 and 2 exemplify state-of-the-art heat recovery systems for removing moisture from flue gases by direct condensation in which a portion of the condensate is evaporated into the combustion air until it is nearly saturated. As shown in FIG. 1, the flue gases are cooled by a direct water spray in a condenser-scrubber. A portion of the condensate is discarded through a drain and the remaining portion is pumped to a humidifying air heater where it is sprayed into the combustion air, thereby heating and humidifying the combustion air to increase its dew point as well as its total enthalpy, resulting in a higher dew point flue gas so that more water vapor can be condensed in the condensing boiler. The cooled excess condensate is then recycled to the condenser-scrubber. Once a steady state is established, the discarded condensate is equal to the amount of water condensed from the flue gases.
As shown in FIG. 2, the condenser and humidifying air heater are integrated into a single device, where the cool combustion air removes heat from the flue gases, causing moisture to condense on the outer surface of a porous membrane. The moisture permeates through the membrane and evaporates into the combustion air, raising its dew point and increasing the inventory of moisture in the system, thereby allowing more heat to be removed. Although simpler than the system shown in FIG. 1, this method does not allow as much control over the condensation and evaporation rates. In addition, as in the system shown in FIG. 1, all of the condensed water is ultimately discarded.